Adsorption of Phosphorus Using Cockle Shell Waste

: Phosphorus is an essential nutrient for aquatic plants and animals. The acceptable range for phosphorus in water is from 0.01 to 0.03 mg/L. However, excessive phosphorus use can result in biodiversity loss and pollution and endanger aquatic creatures and human health because the pollutants are non-biodegradable and thus accumulate over time. This work investigated the removal of phosphorus from synthetic wastewater containing KH 2 PO 4 via adsorption using calcined cockle shell waste. Phosphorus adsorption by calcined cockle shell waste of less than 75 μm particle size was investigated. Five different adsorbent dosages (0.2, 0.4, 0.6, 0.8, and 1.0 g) mixed with 10 ppm phosphate were adsorbed for 60 min. The presence of calcined cockle shell waste was determined by using scanning electron microscopy (SEM), Fourier - transform infrared spectroscopy (FTIR), and a DR6000 UV – visible spectrophotometer. In brief, the highest dosage of 1.0 g removed 94.96% phosphorus from the synthetic wastewater, while the longest treatment time resulted in 95.74% phosphorus adsorption. The proposed method is low-cost and convenient.


Introduction
Agriculture has increased the amount of phosphorus discharged into surface water. Crop fertilisation releases phosphorus into the nearby aquatic ecology, where it is a vital nutrient that promotes the growth of biological organisms and algae [1]. The permissible limit of phosphate in water is between 0.01 and 0.03 mg/L [1,2]. However, the nonstop demand economy of agriculture has caused excess phosphorus discharge, leading to environmental degradation, including water pollution. The uncontrolled amount of phosphorus discharged accelerates eutrophication and algal blooms. This situation causes an increase in the biochemical oxygen demand (BOD) and the failure of aquatic ecosystems, which endanger human health and other organisms.
Cockleshells (Anadara granosa), which live behaviourally buried in the sediment, have grown in importance as a valuable ecological marine product in Malaysia, particularly on the west coast of West Malaysia. Cockles contribute to about 93% of marine aquaculture production [3,4]. The local cockle shells, 'kerang', are classified under the phylum Mollusca, class Bivalvia, order Arcoida, and family Arcidae [5]. Cockles are an affordable source of protein and are broadly distributed in Southeast Asian countries [6]. They produce an abundance of shell waste that is dumped into landfills or along the seashores due to the expensive disposal procedure. The high amount of untreated cockle shell waste leaves an unpleasant odour and visual disturbance to the surroundings. Excessive amounts of phosphorus can be treated with the adsorption method [1]. This process is advantageous as it is easy to handle. Calcined cockle shell waste contains more than 95% calcium carbonate (CaCO3) by weight. They can be used as a low-cost adsorbent material as they have shown good performance for the removal of organic and inorganic matters in polluted water [6]. Through calcination or thermal decomposition, CaCO3 is converted to calcium oxide, CaO. CaO is an alkaline earth oxide and it can be used for industrial wastewater and sewage treatment.
The primary goal of this research is to assess the performance of the powdered form of calcined cockle shell for phosphorus removal. Phosphorus-rich synthetic wastewater was treated with the calcined sample under an appropriate temperature to investigate the effects of different dosages and treatment times. Cockleshell waste has the potential to be used as an adsorbent for removing phosphate (PO4 3-) in polluted water, thereby reducing a source of eutrophication. In addition, the removal of phosphorus from water bodies benefits the environment.

Adsorbent
Cockle shells (Anadara granosa) were collected from Teluk Kerang, Pontian. The cockle shells were cleaned using tap water and brushes to remove any debris. Then, the shells were boiled for 10 min at 100 °C before being air-dried at room temperature. The cockle shells were then dried in a drying oven at 110 °C for 2 h to ensure complete drying before being crushed with a pestle and mortar. The shell was ball-milled using the dry-milling method for 45 min at 500 rpm. Subsequently, the resultant microsized powder was sieved using a stainless steel laboratory filter with an aperture of 75 µm. Finally, the powder was calcined in a chamber furnace and stored in a ziplock bag [7,8]. Figure 1 shows the overall process of adsorbent preparation. crushing, (f) grinding, and (g) sieving.

Adsorbate
To produce the synthetic solution to test the adsorption process, 0.1433 g of anhydrous potassium phosphate (KH2PO4) was dissolved in 1 L of deionised water (DI) to make a 100 ppm stock solution in a volumetric flask. Then, 500 mL of the stock solution was diluted with 500 mL of DI in another volumetric flask, bringing the concentration to 50 ppm. Finally, in a volumetric flask labelled as 10 ppm, 200 mL of a 50 ppm solution was diluted with 800 mL of DI to make a phosphate solution at the desired experimental concentration of 10 ppm.

Chemicals
Molybdate and amino acid were used to measure the concentration of the various phosphate solutions. The concentration of the phosphate solutions was determined using a DR6000 UVvisible spectrophotometer and the reagents.

Characterisation
The calcined cockle shells (CCS) were subjected to various analyses. The image of the cockle pores before and after calcination was captured using SEM (EM-30AX Plus). The functional group was determined using Fourier-transform infrared spectroscopy (FTIR; PerkinElmer Spectrum Two™ IR spectrometer). A DR6000 UV-vis spectrophotometer (Hach DR6000 Benchtop Spectrophotometer) was used to figure out the initial and final concentrations of the adsorbate).

Adsorption studies
The best method for removing phosphorus from synthetic wastewater is via the adsorption process. After the process of adsorption in a orbital shaker, the adsorbate and adsorbent were separated using filter paper. The adsorbate of 25 mL of potassium phosphate was added with 1 mL of molybdate reagent and 1 mL of amino acid reagent. The final concentration of phosphorus solution was measured using the UV-vis spectrophotometer ( Figure 2). The percentage of removal efficiency, as well as the phosphorus sorption capacity, were calculated.

Effect of Different Amounts of Dosage of Adsorbent
Various adsorbent dosages were tested to determine the best dosage for phosphate adsorption in synthetic wastewater. Different dosages of CCS adsorbent (0.2, 0.4, 0.6, 0.8, and 1.0 g) was added to a set of conical flasks containing 100 mL of 10 ppm phosphate solution. The solutionadsorbent mixtures were then incubated for 60 min in an incubator shaker set to 37 °C and 150 rpm [9]. The phosphate removal effectiveness (E) and sorption capacity (q) of various adsorbent dosages were calculated using Eqs. 1 and 2.
where E stands for removal efficiency. Ci denotes the initial phosphate concentration (mg/L) and Cf denotes the phosphate concentration after adsorption (mg/L).
where q is adsorption capacity (mg/g). Ci is the initial phosphate concentration (mg/L). Cf is the final phosphate concentration in the solution (mg/L), m is adsorbent mass (g), and V the volume of the solution (L).

Effect of Treatment Time
The treatment time was varied to determine the best time for phosphorus adsorption in the synthetic wastewater solution. Various durations of 20, 30, 40, 50, and 60 min were used for the test. The other experimental conditions were kept constant with a 10 ppm initial concentration of phosphate solution and 1.0 g of CCS for adsorbent dosage. The samples were placed on an incubator shaker with a 150 rpm agitation at a constant temperature of 37 °C.

The Calcination Process of Cockle Shell
The cockle shells were weighed using an analytical balance before the calcination process. Usually, the temperature range for calcination is 700-900 °C to obtain the perfect porosity. The porosity makes the adsorption more efficient since it increases the surface area of the adsorbent. The cockle shells were calcined in a chamber furnace (Carbolite Gero 30-3000 °C) at 900 °C for 2 h [10]. The calcination process used a heating rate of 27 °C/min. Before being placed in the furnace chamber, the powder was poured into the crucible. The weight of the calcined cockle shell (CCS) powder decreased. The purpose of the calcination process was to convert CaCO3 from the crab shell to CaO. Eq. 3 shows the reaction undergone by CaCO3 during the calcination process, in which it is converted to CaO. The porosity makes the adsorbents have more surface area, which makes the percentage of adsorption go up.

→ + 2
Eq. 3 This equation shows how CaCO3 changes after being heated and calcium carbonate transforms into calcium oxide.

Fourier-Transform Infrared Spectroscopy (FTIR)
FTIR was conducted to determine the presence of functional groups of CaCO3 in the raw cockle shell sample and CaO in the calcined cockle shell sample. Figure 3 shows the bands in the IR spectra with an absorption range of 400-4000 cm −1 . Different types of bonds are commonly absorbed in their range of region since the function of the FTIR is to identify the functional group.   cm −1 after adsorption with dosage analysis, and 711.70 and 871.42 cm −1 after adsorption with treatment time analysis. These bands are related to the symmetric stretching modes of vibration and out-of-plane bending of vibration modes of CO3 2− , respectively. The classification of the -OH band appearing at 3641.07 cm −1 is the alcohol group with strong and broad intensity. This peak appears much broader than the other IR absorptions in the spectrum.

Scanning Electron Microscopy (SEM)
SEM was used to observe the surface morphology of raw cockle shells (RCS) and calcined cockle shells (CCS). It shows the surface texture and porosity of the adsorbent to determine the surface availability for phosphorus adsorption on the adsorbent. Figure 4 (a) shows the SEM images of RCS, while Figure 4 (b) shows the SEM images of CCS. The average diameter of the cockle shell powder particles was 10 µm, indicating the availability of a net-work of porous surface textures that results in higher surface areas for phosphorus adsorption. The RCS micrograph shows a combination of stone and fine powder particles. On the other hand, the CCS micrograph illustrates a coral shape with spaces that grew larger as a result of the high calcination temperature. These spaces or porosity may be capable of trapping adsorbates during the physical adsorption process.

Effect of different adsorbent dosage
Different adsorbent dosages were used to study the removal efficiency of phosphorus (E) and the adsorption capacity for phosphorus (q) from the synthetic wastewater solution. The adsorbent dosages used throughout the experiment were 0.2, 0.4, 0.6, 0.8, and 1.0 g. As shown in Figure 5, the phosphorus removal efficiency by CCS was proportional to the adsorbent dosage. As the adsorbent dosage increased from 0.2 to 1.0 g, the phosphorus removal efficiency by CCS increased from 87.65 % to 95.96 %. The higher removal percentage indicates that more phosphorus is attached to the adsorbent surface at that dosage. In essence, a higher adsorbent dosage results in more adsorption sites being available [12]. However, when adsorbent dosage was increased from 0.2 to 1.0 g, the phosphorus adsorption capacity of CCS decreased from 5.04 to 1.09 mg/g, as shown in Figure 5. According to the findings, the adsorbent has a consistent number of active adsorption sites at lower concentrations, implying that the adsorption site may adsorb more metal ions as shown in surface morphology ( Figure 4). As the concentration increases, active sites may become saturated, causing the adsorption capacity to decrease. Surface saturation is one of the sorption properties that can influence the initial metal ion concentration in the solution.

Effect of Treatment Time
The was initially quite fast, and longer contact times resulted in higher phosphorus removal, as shown in Figure 6. Phosphorus adsorption increased from 88.43 % to 95.74 % between 20 and 60 min. The increase in phosphorus adsorption may be due to the longer the contact period, which led to the availability of more adsorption sites on the adsorbent surface to adsorb the phosphate anions in the solution [13]. In brief, CCS is an ideal substrate for increasing the rate of phosphorus removal. When the duration of adsorption increased from 20 to 60 min, the phosphorus adsorption capacity of CCS increased from 1.02 to 1.10 mg/g.

Conclusions
CCS acted as a natural adsorbent and effectviely removed phosphate compounds from the synthetic wastewater solution. In this work, the calcined cockle shell powder with the highest dosage of 1.0 g removed 94.96 % phosphorus from the synthetic wastewater. Meanwhile, the treatment time of 60 min resulted in 95.74 % phosphorus adsorption. It is shown the potential of adsorbents to remove phosphate from water. Adsorption process using cockle shell waste is advantageous due to its low cost; therefore, further research of cockle shell adsorption may be useful and can potentially benefit the environment.